If your house is heated Danios will be fine without a heater as they are temperate fish. If the tank temperature stays above 16C they'll have no problems. When you replace the heater, note the temperature of the water and set the heater only a degree higher. Wait 12 hours and raise by 1 more degree and so on as rapid changes in temperature are harmful. 20C is the best temperature to maintain a Danio tank at.